"Ozymandias" is an ancient Greek name for Ramses II of Egypt. It is actually a Greek version of the Egyptian phrase "User-maat-Re," one of Ramses's Egyptian names. Why not just call the poem "User-maat-Re," you might ask? Well, this is Shelley, who had studied ancient Greek; it is therefore no surprise that he chooses to use the Greek name "Ozymandias," rather than the Egyptian name.
Ramses II was one of Egypt's greatest pharaohs, and many of the most famous tourist sites in Egypt, including the temple of Abu Simbel and the Ramesseum in Thebes, were built or planned during his incredibly long tenure (he lived until he was 90!). He is known not only for his building program, but also for several ambitious foreign military campaigns and for his diplomacy, especially with the Hittites, another important ancient people.